Umbrella base



FIG. 1 is a perspective view of an umbrella base shown with environment;

FIG. 2 is a perspective view of the umbrella base shown without environment;

FIG. 3 is a top view thereof;

FIG. 4 is a bottom view thereof; and,

FIG. 5 is a right side view thereof; the left, front, and rear views being identical to the right side view.

The broken lines depict portions of the umbrella base and environment that form no part of the claimed design. 

CLAIM The ornamental design for an umbrella base, as shown and described. 